Field of Invention
The present invention is in the technical field of energy storage devices. More particularly, the present invention is in the technical field of rechargeable batteries using iron electrodes.
Related Art
The nickel iron (Ni—Fe) battery was independently developed by Edison in the United States and by Junger in Sweden in 1901. It was industrially important from its introduction until the 1970's when batteries with superior specific energy and energy density replaced Ni—Fe batteries in many applications.
However, Ni—Fe batteries have many advantages over other battery chemistries. The Ni—Fe battery is a very robust battery which is very tolerant of abuse such as overcharge and overdischarge and can have a very long life. It is often used in backup situations where it can be continuously trickle-charged and last more than 20 years. Additionally, the iron active material is much less expensive than active materials used in other alkaline battery systems such as NiMH or in non-aqueous batteries such as Li Ion. However, the low specific energy, low energy density, and poor power have limited the applications of this battery system.
The Ni—Fe battery is a rechargeable battery having a nickel(III) oxy-hydroxide positive electrode and an iron negative electrode, with an alkaline electrolyte such as potassium hydroxide. The overall cell reaction can be written as:2NiOOH+Fe+2H2O2Ni(OH)2+Fe(OH)2  (1)
Rechargeable batteries often require several charge-discharge cycles prior to achieving optimum performance. During these early cycles, critical surface films are formed on the electrode surfaces that affect the performance of the cell during later cycling. These early cycles are commonly termed formation cycles in the battery industry. In the case of nickel-iron batteries (Ni—Fe), 30 to 60 formation cycles are typically needed to achieve the full capacity of the cell. Formation cycling sometimes requires cycling at varied temperature regimes which complicates the process. This formation process is expensive, time consuming, consumes electrolyte which needs replacing, and generates a significant amount of gas. Therefore, reducing the number of formation cycles and simplifying the formation process is a worthy goal.
Manohar et. al. in “Understanding the factors affecting the formation of Carbonyl Iron Electrodes in Rechargeable Alkaline Iron Batteries”, J. Electrochem. Soc., 159, 12, (2012) A 2148-2155, reported that one reason for the long formation time could also be the poor wettability of the iron electrode and the inaccessibility of the pores of the iron by the electrolyte. As the pores became more accessible the charge and discharge process produced a progressively rougher surface resulting in an increase in electrochemically active surface area and discharge capacity. Triton X-100, a surfactant, reduced the number of cycles required to achieve higher capacity presumably because it improved access of the electrolyte to the pores.
U.S. Pat. No. 3,507,696 teaches that a mixture of FeO and Fe2O3 powders fused with sulfur at 120° C. yields an active material that may be used in an aqueous slurry to impregnate sintered nickel fiber plaques that can used as a negative electrode in a Ni—Fe battery. Several formation cycles are needed to achieve high capacity.
It would be of benefit to the industry to have a battery comprising an iron electrode which is conditioned prior to any charge-discharge cycle so as to minimize the need for formation cycles.